A cavity resonator is a hollow conductive block where an electromagnetic standing wave can be supported. Cavity filters use the principle of a cavity resonator to filter out undesired frequency components and obtain desired frequency components. Cavity filters are often low loss and highly linear that they are often used for pre-select filters in wireless base stations and remote radio heads.
One of the conventional approaches for a compact cavity filter is to use an evanescent mode cavity resonator. The evanescent cavity resonator, which is sometimes called a transverse electromagnetic mode (TEM) cavity filter, achieves smaller than wavelength resonance for a given cavity size by introducing a strongly capacitive element within the cavity resonator, such as a metal post/disk close to one of the metal walls of the cavity. Unfortunately, such compact cavity filters are sensitive to changes in temperature as temperature changes cause the dimensions of the cavity filter to change by thermal expansion or contraction. This affects the frequency characteristics of the cavity filter. One of the current approaches to cavity filter design involve using materials with small thermal coefficients of expansion so that the constructed filter dimensions are relatively insensitive to changes in temperature.